Urethane polymers have in the past been modified to modulate their finctionality by endcapping some or all of the isocyanate groups with a variety of organosilanes to yield silane end-capped urethane polymers containing minimal or no isocyanate groups. For example, in U.S. Pat. No. 3,632,557 to Brode and Conte, the complete end-capping of polyurethanes with primary and secondary aliphatic aminosilanes was disclosed, yielding silane-terminated polymers that were curable at room temperature. In U.S. Pat. No. 3,979,344, Bryant and Weis disclosed that a small amount of 3-(N-2-aminoethyl)-aminopropyltrimethoxysilane blended with vulcanizable silane-terminated polyurethanes yielded room-temperature-curable silane-terminated sealant compositions having improved cure speeds, useful in metal and glass primer compositions. Bryant et al., in U.S. Pat. No. 4,222,925, disclosed the same composition as that set out in U.S. Pat. No. 3,979,344, with the addition of a high strength reinforcing carbon black filler, and optionally, a moisture curing catalyst such as dibutytin diacetate.
In U.S. Pat. No. 4,374,237 to Berger, et al., curable isocyanate-terminated polyurethanes having at least some of the terminal isocyanate groups reacted with a secondary amine containing silane monomer having two trialkoxy silane groups were described. The compounds were disclosed as being useful in sealant compositions having enhanced wet adhesion.
Other silane end-capped urethane polymers and sealants were disclosed in U.S. Pat. No. 3,627,722 to Seiter, which described polyurethane sealants such as alkylaminoalkyltrialkoxysilanes, mercaptoalkyltrialkoxysilanes, and arylaminoalkyltrialkoxysilanes containing a significant percentage, but preferably less than all, of terminal isocyanate groups endblocked with--Si(OR)3, where R was a lower alkyl group; in U.S. Pat. No. 4,067,844 to Barron and Turk, which disclosed curable poylurethanes in which a portion of the NCO terminations are reacted with certain amino silanes (or with the residue on reaction of a mercaptosilane with monoepoxide, or with the residue on reaction of an epoxysilane with a secondary amine); in U.S. Pat. No. 4,345,053 to Rizk, et al., which disclosed a moisture-curable silane-terminated polymer prepared by reacting a polyurethane having terminal active hydrogen atoms with an isocyanato organosilane having a terminal isocyanate group and at least one hydrolyzable alkoxy group bonded to silicon; and in U.S. Pat. No. 4.625,012 to Rizk and Hsieh, which disclosed a moisture-curable polyurethane having terminal isocyanate groups and silane groups having at least one hydrolyzable alkoxy group bonded to silicon, in which the silane groups may be pendant to the chain.
Silane-endcapped urethane sealants frequently exhibit insufficient flexibility to be useful in applications requiring considerable extension and compression. To overcome these problems, U.S. Pat. No. 4,645,816 to Pohl and Osterholtz described a novel class of room-temperature, moisture-curable, silane-terminated polyurethanes bearing terminal isocyanate groups reacted with a silane monomer having one dialkoxy silane group and an organo-functional group with at least one active hydrogen. The polymers were crosslinked to produce elastomeric networks with improved flexibility.
Another approach to reducing the crosslinking density of the cured elastomers, is to use secondary aminosilanes with bulky substituents on the nitrogen as silane endcappers, preferably reacting all free isocyanate endgroups with these secondary amino silanes. Feng reported in European Patent Application No. 676,403 that the use of arylaminosilanes, particularly having one dialkoxy silane group provided the added benefit of further improved flexibility. Zwiener, et al. disclosed in U.S. Pat. No. 5,364,955 similar benefits using certain N-alkoxysilylalkyl-aspartic acid esters.
The use of difunctional silanes and/or sterically hindered silanes, typically employing amine reactivity for the endcapping of the urethane prepolymers, suffers from several drawbacks. The secondary amine containing silanes are slow to react with the urethane prepolymer while polymer endcapped with dialkoxyfunctional silanes are typically very slow to cure. Particularly the formation of urea which is experienced when using amino silanes leads to a meaningful increase in viscosity of the prepolymer, potentially resulting in processing problems and application restrictions.
Employing trialkoxy silane groups as endcappers for silylated precursors intended to have considerable flexibility has heretofore required forming extended polymer chains with a significantly high average molecular weight to balance out the crosslinking density inherent in the use of the trifunctional endcapper. This however has led to increased viscosity of the eventual product, to unacceptably high levels particularly using amino silane endcappers building urea bonds. The synthesis routes to build up chain length via the polyurethane polyaddition reaction using conventional polyether polyols have exhibited the problem of low to negligible residual functionality of the urethane prepolymer before silane endcapping. Hence, synthesis of these types of systems may be not feasible, and/or these systems may offer unacceptable cure profiles and mechanical properties.
Thus, there remains a need for silylated polyurethane precursors that offer simultaneously favorable viscosities, faster cure, and significantly improved mechanical properties, flexibility, and no residual tack after final cure.